Re: [PATCH v3 2/2] sched: consider missed ticks when updating global cpu load

[Byungchul Park]

On Fri, Oct 02, 2015 at 05:59:06PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 02, 2015 at 04:46:14PM +0900, byungchul.park@xxxxxxx wrote:
> > From: Byungchul Park <byungchul.park@xxxxxxx>
> > 
> > in hrtimer_interrupt(), the first tick_program_event() can be failed
> > because the next timer could be already expired due to,
> > (see the comment in hrtimer_interrupt())
> > 
> > - tracing
> > - long lasting callbacks
> 
> If anything keeps interrupts disabled for longer than 1 tick, you'd
> better go fix that.

tracing and long lasting callbacks are not my case.

> 
> > - being scheduled away when running in a VM

this is my case.

> 
> Not sure how much I should care about that, and this patch is completely
> wrong for that anyhow.

do you mean, in upper case, we must assume ticks occur with 1 tick interval
when update_process_times() is called even though more than 1 tick is
actually passed in host? right? i am really wondering it. i would admit i
was wrong if what you mean is same as what i understand.

> 
> And this case in hrtimer_interrupt() is basically a fail case, if you
> hit that, you've got bigger problems. The solution is to rework things
> so you don't get there.
> 
> 
> > in the case that the first tick_program_event() is failed, the second
> > tick_program_event() set the expired time to more than one tick later.
> > then next tick can happen after more than one tick, even though tick is
> > not stopped by e.g. NOHZ.
> > 
> > when the next tick occurs, update_process_times() -> scheduler_tick()
> > -> update_cpu_load_active() is performed, assuming the distance between
> > last tick and current tick is 1 tick! it's wrong in this case. thus,
> > this abnormal case should be considered in update_cpu_load_active().
> 
> Everything in update_process_times() assumes 1 tick, just fixing up 
> one function inside that callchain is wrong -- I've already told you
> that.

anyway, it's wrong for update_process_times() to assume 1 tick because
tick_irq_exit() -> tick_nohz_irq_exit() -> tick_nohz_full_update_tick()
-> tick_nohz_restart_sched_tick() can happen at full NOHZ as i already
said. in this full NOHZ case for tick to restart from non-idle,

1. update_process_times() -> account_process_tick() must be able to handle
more than one tick, or tick_nohz_restart_sched_tick() should handle the
case additionally. (i think the latter is better.) i will try to modify
the code to handle it if you agree with me.

2. to handle full NOHZ, tick_nohz_restart_sched_tick() should call
update_cpu_load_active() instead of update_cpu_load_nohz() with my 1/2
patch and 2/2 patch, or we should modify update_cpu_load_nohz() to know
full NOHZ, which currently don't know full NOHZ. (you may agree with the
latter.) in any case, 1/2 patch is necessary which current code is
absolutely missing.

peter, what do you think about my opinion? and about my 1/2 patch?
i will modify 2/2 patch depending on your feedback.

> 
> 
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